Electrical equipment or appliance connected to AC power grid line should satisfy current harmonic standard IEC61000-3-2. With regard to different equipment or applications, IEC61000-3-2 has correspondingly set different current harmonic limits, among which, a Class A limit is for normal electrical equipment, Class B for portable tools and non-professional welding equipment, Class C for lighting equipment, and class D for portable personal computer, monitor and TV.
Existing switching mode power supply technology for realizing power factor correction function mainly utilizes active power factor correction (PFC) method implemented by boost converter (see FIG. 1 and FIG. 2).
Such boost converter when integrated with DC-DC converter, can achieve excellent power factor correction performance. However, due to independent circuits of the boost and DC-DC converters, each circuit includes its individual power switching component, and feedback control and driving unit, so that the cost is high, size becomes big and efficiency exhibits low.
In order to overcome the above shortcomings, a combination circuit of boost and bridge type converters is presented in FIG. 3. This circuit, by taking use of the switching component of the bridge type converter, to also drive the boost inductor, eliminates the use of a boost switching component, a boost rectifier and an individual pulse width modulation (PWM) PFC control unit in a conventional boost PFC circuit, as the result, cost is reduced, space saved and efficiency improved.
However, the above combination power supply (FIG. 3) shares a common switching component, making it impossible to simultaneously control the boost converter and the DC-DC converter by using a conventional feedback control and driving unit. If a conventional feedback control and driving unit is used to control such a combination power supply, there could be a number of disadvantages:
Disadvantage 1: Boost output voltage not under control.
Disadvantage 2: When boost inductor operates at a high AC input voltage, due to the low reset voltage Vdc−Vin (Vdc: boost output voltage on C2), an unlimited duty decided by the DC-DC converter would cause insufficient magnetic reset of the boost inductor, and Vdc has to be designed high enough to ensure the reset.
Disadvantage 3: When boost inductor operates at a continuous current mode, first switching component Q2 works in hard switching mode, and loss becomes high.
Disadvantage 4: When boost inductor operates at a discontinuous current mode, boost inductor L1, boost capacitor C1, second switching component Q3, storage capacitor C2 construct a resonant circuit which may cause uncontrolled current loop loss.
Causes of the disadvantages are explained as follows:
According to FIG. 3 and FIG. 4, the combination power supply of boost and asymmetrical half bridge converters operates as follows: The bridge type DC-DC converter is controlled by PWM. Sense and feedback the DC-DC converter's output parameter like voltage or current or power, compare the output parameter to a setting value and according to automatic control theory (Examples: PID, Zero pole method), obtain the duty of PWM. Form a PWM chopping signal under a preset frequency which is usually set by a control IC, and use the PWM chopping signal to drive first switching component Q2 and second switching component Q3. The DC-DC converter's output can be stabilized by adjustment of the obtained duty.
When first switching component Q2 conducts, second switching component Q3 cuts off, storage capacitor C2, first switching component Q2, resonant capacitor Cr, resonant inductor Lr and main transformer T1 construct a primary side power loop of serial resonant half bridges, and energy transfers to secondary side through T1. At same time, Boost capacitor C1, first switching component Q2 and boost inductor L1 construct a boost loop, and boost energy is stored in L1.
When first switching component Q2 cuts off, second switching component Q3 conducts, storage capacitor C2, second switching component Q3, resonant capacitor Cr, resonant inductor Lr and main transformer T1 construct a primary side power loop of serial resonant half bridges, and energy continuously transfers to secondary side through T1. Induced voltage on boost inductor L1 and voltage on boost capacitor C1 accumulate to charge storage capacitor C2, accomplishing boost conversion.
Controlling and operating above combination power supply may lead to the following disadvantages:
For disadvantage 1: Boost output is not under control. By using feedback and driving controller 300, first switching component Q2 and second switching component Q3 are driven by PWM complement signal, so that DC-DC output can be stabilized by duty of PWM signal as in conventional method. However, due to the use of the same PWM signal to drive boost circuit via Q2, boost circuit output has no feedback control and it swings according to DC-DC's PWM and input AC instant voltage. Bridge type DC-DC converter's maximum duty is always smaller than 50%, and therefore, boost converter's duty is also limited to less than 50%. Instead of a nearly 100% operation in conventional boost converters, 50% duty at input AC low instant voltage makes the boost converter insufficient to convert power, so that the boost output voltage on C2 would be possibly lower than input AC peak voltage. Charging current through D1 might occur at peak AC instant voltage as shown in FIG. 5a and FIG. 5b which makes AC input current distorted at low input voltage and heavy load.
For disadvantage 2: When boost inductor operates at high AC input voltage, due to low reset voltage Vdc−Vin (Vdc: boost output voltage on C2; Vin: instant AC input voltage), unlimited duty decided by the DC-DC converter would cause insufficient magnetic reset of the boost inductor.
Magnetic reset equation of boost inductor is expressed as Vin*Duty=(Vdc−Vin)*(1−Duty), so Duty(max)=(Vdc−Vin)/Vdc can be derived. When Vin is at its sinusoidal peak, due to instant Vin close to Vdc, Duty(max) has to be very small to ensure voltage-second balance to realize magnetic reset. Once DC-DC converter's duty exceeds Duty(max), boost inductor L1 will be saturated and first switching component damaged.
For disadvantage 3: When boost inductor operates at continuous current mode, first switching component Q2 works in hard switching mode as that in conventional boost converters. During cut-off interval of Q2, there is no resonant current reversely discharging the parasitic capacitor of Q2, so zero voltage switch (ZVS) would not occur at Q2's subsequent turning on.
For disadvantage 4: When boost inductor operates at discontinuous current mode, boost inductor L1, boost capacitor C1, second switching component Q3, storage capacitor C2 construct a resonant circuit. After boost current falls to zero ampere, and at the state of Q2 turn off and Q3 turn on, voltage on storage C2 will charge resonant tank constituted by L1 and C1 through Q3. The resonant current causes current loop loss if it was not under control.
For purpose of solving the above problems, a kind of PFWM method is disclosed to replace the conventional PWM or PFM control method.